#wrap {
  width: 800px;
  height: 400px;
  display: flex;
  flex-flow: row wrap;
  align-content: flex-start;
  border: 1px solid dimgray;
  box-sizing: border-box;
}
#wrap .box {
  width: 25%;
  height: 50px;
  border-right: 1px solid dimgray;
  border-bottom: 1px solid dimgray;
}
